Electrical capacitance instrument and manufacturing method of the same

Abstract: An electrical capacitance instrument achieving high-volume productivity and a manufacturing method for the same. In a slide calipers detecting a change of electrical capacitance upon a displacement value of the slider 10 in relation to the base scale 1 having the main scale 20 of the measuring standard, the improved slide calipers is structured by the main scale 20 defined by plural electrodes 21 printed on a surface of the base plate 22 made of an insulator film at every certain intervals with electrically conductive ink, and the base scale 1 made by means of an injection molding using a material of engineering plastics on which the main scale 20 is applied securely, so that the same preferable slide calipers can be manufactured in large numbers.

Description: This invention is broadly concerned with an electrical capacitance instrument and a manufacturing method of the same and intended particularly to be used to improve a high-volume productivity of electrical capacitance instrument and a manufacturing method thereof. Conventionally, in the art of measuring instrument such as a slide calipers, there has been employed an electronic one having a digital display to prevent or decrease artificial mistakes when reading out measured value with the eye. The conventional electronic measuring instrument is generally defined by a main body and a measuring element being adapted to move in relation to the main body. The instant displacement value of the measuring element is transformed into electrical signals by an conventionally known electrical capacitance instrument to be displayed in numerical values. The electrical capacitance instrument includes a main scale which has a plurality of electrodes thereon arranged at a fixed spacing from one another and a measuring element which has electrodes corresponding to the electrodes provided on the mains scale in a statically relating state. The displacement value of the measuring element is transformed into electrical signals based on the electrical capacitance changed in accordance with the displacement value of the measuring element. Incidentally, the main scale is generally manufactured separately from a main body of the instrument and then secured on the main body by means of a bonding agent. However, the manufacturing process for the main scale does not attain a high-volume productivity due to the number of steps involved, that is, a step for evaporating metal membrane on a base plate made from epoxy-glass having insulation properties, a step for masking necessary area on the metal membrane, a step for forming plural electrodes by etching unnecessary area on the metal membrane, and a step for forming an insulation membrane to cover surface of the electrodes. It would therefore be difficult to achieve a preferable high-volume productivity of the measuring instrument having such a main scale even though the productivity of other parts except the main scale will be improved. Accordingly, an object of the present invention is to provide an ideal electrical capacitance instrument in preferable high-volume productivity and a manufacturing method of the same. IBM Technical Disclosure Bulletin vol. 21, no. 1, June 1978, pages 242-243 discloses an electrical capacitance instrument having a main scale on a main body and a measuring element which is arranged to move in relation to the main body to provide a measurement of a displacement value of the measuring element relative to the main body; wherein the main scale is provided by an insulating base plate, and a plurality of electrodes printed with an electrically conductive ink on the base plate at intervals along the direction of movement of the measuring element; and according to a first aspect of the present invention such an instrument is characterised by an insulating bonding layer formed over the electrodes and between the base plate and the main body and bonding the main scale to the main body. When printing plural electrodes on the main body directly, the main body is made of a material having insulation properties and the plural electrodes are shielded by a coating layer made of a material having insulation properties. The main scale is applied to the main body by means of a bonding layer provided on a surface to which the plural electrodes of the base plate have already been printed. Incidentally, the main body uses a material having insulation properties, particularly engineering plastics (e.g., a synthetic resin of polybutylene terphthalate) as its base material and the base material may further include minerals such as kalium titanate. The base plate is preferably made of a film having plasticity. The base plate can be made of a transparent material and the slide calipers may further include a decorative printing layer to print thereon characters or patterns between the one surface of the base plate and an electrode layer printed with the electrodes. The main body may be a base scale having at least one measuring jaw, and wherein the measuring element is adapted to move along the base scale in a sliding contact state with the base scale and has at least one measuring jaw corresponding to that of the base scale. According to a second aspect of the present invention a method of manufacturing an electrical capacitance instrument having a main body with a main scale, and a measuring element which is arranged to move in relation to the main body to provide a measurement of a displacement value of the measuring element relative to the main body comprises printing a plurality of electrodes on a base plate made of an insulating film at intervals with an electrically conductive ink to form the main scale; characterised by forming an insulating bonding layer on the base plate and covering the electrodes; and bonding the base plate to the main body by means of the bonding layer so that the bonding layer is between the base plate and the main body. The base plate can be made of a transparent material and the invented method can further include a step for making a decorative printing layer to print characters or patterns on the base plate and printing plural electrodes on the decorative printing layer. Incidentally, the main body can be formed by means of an injection mold using a material having insulation properties. According to the above mentioned invention, as the main body is provided with the main scale having plural electrodes printed thereon by means of an electrically conductive ink, compared to the conventional main scale forming method of forming electrodes thereon by etching, the production of the main scale of the present invention is far easier and simpler. When printing electrodes on one surface of the base plate of an insulator film to obtain the main scale, large numbers of identical scales can be produced and a high-volume productivity of the electrical capacitance instrument can be achieved. When applying the main scale to the main body, the bonding layer is formed by applying a bonding agent on a surface on which the electrodes of the base plate are already painted, the bonding layer produces a smooth surface to which the main scale is applied on the main body and an insulation layer will not be required due to an insulation state between the electrodes. If the base plate is of a film having plasticity, it will be preferable to apply the main scale to the main body securely. If the base plate is made of a transparent material and the decorative printing layer is additionally intervened between the base plate and the electrode layer, the painted characters or patters on the decorative printing layer can be read out through the base plate. If the material for the main body is a material having insulation properties, particularly engineering plastics capable of being molded by injection, a large number of the same main bodies can be produced. Furthermore, the main body itself becomes an insulation member, so that it is not necessary to prepare an insulation layer additionally between the main body and the main scale to provide sufficient insulation. Accordingly, a high-volume productivity of the electrical capacitance instrument can be achieved. In the accompanying drawings: FIG. 1 is a front view of the appearance of the overall composition of a slide calipers in the first embodiment of the present invention; FIG. 2 is an enlarged plane view of a main scale in the first embodiment; FIG. 3 is an enlarged sectional view of the main scale; FIG. 4 is a view depicting an electrical capacitance encoder in the first embodiment and FIG. 5 is a plane view depicting a modification of the first embodiment. Certain terminology will be used in the following description for convenience in reference only and will not be limiting. The words "up", "down", "right" and "left" will designate directions in the drawings to which reference is made. The words "in" and "out" will refer to directions toward and away from, respectively, the geometric center of the device and designated parts thereof. Such terminology will include derivatives and words of similar import. The preferred embodiment of the present invention will now be described with reference to the drawings. An example according to the present invention will hereunder be described with reference to FIGs. 1 - 4. The following example discloses a slide caliper in which an electrical capacitance instrument according to the present invention is preferably utilized. The slide caliper is, as shown in FIG. 1, defined by a base scale 1 of a main body laterally extending on the drawing and a slider 10 of a measuring element movable into a longitudinal direction on the base scale 1. The base scale 1 is made by an injection molding method by means of engineering plastics having sufficient insulation properties. The engineering plastics uses a synthetic resin of polybutylene terphthalate (hereinafter referred to "PBT") resin as a base material and additionally includes mineral such as kalium titanate to have stiffness and wear-resistance properties and lightness in weight compared with steel. The weight of the base scale 1 is lightened in weight by about one fifth of that made of metal. As can be seen from the drawing, there is attached integrally to a left end most portion of the base scale 1 an inboard measuring jaw 2 extending upwardly on the drawing and an outboard measuring jaw 3 extending oppositely or downwardly on the drawing. The base scale 1 is provided with a main scale 20 in a state that the front surface of the base scale 1 is entirely covered with the main scale 20. The main scale 20 has at a fixed spacing from one another plural lattice electrodes 21 defining an electrical capacitance encoder 40. Now, the slider 10 is produced by means of the same material (engineering plastics) and the same method (injection molding method) as those in the base scale 1. The slider 10 integrally has at the left end most thereof an inboard measuring jaw 12 and an outboard measuring jaw 13 to correspond to the inboard measuring jaw 2 and the outboard measuring jaw 3 of the base scale 1. In an enclosed space at the front side of the slider 10, there are received a digital display 14 to display an instant displacement value of the slider 10 in digital, a power-off switch 15, a switch 16 served both as a power-on switch and a zero-setting switch and so on. Now referring to FIGs. 2 and 3, the main scale 20 adapted to put on the base scale 1 is depicted in an enlarged state. The main scale 20 should be understood as to be finished with a printing of electrodes 21 on a front surface of the web-shaped base plate 22. The main scale 20 has a multi-layer construction defined by a decorative printing layer 23, an electrode layer 24 and a bonding layer 25, each layer lying one above the other in this order or downwardly from the surface of the base plate 22 as shown in FIG. 3. The base plate 22 is made of polyester transparent film having a thickness of 0.125 mm and plasticity properties and is formed into a predetermined configuration. The decorative printing layer 23 is a membrane of the ink at a thickness of about 0.125 mm and is formed by being directly put on the surface of the base plate 22. The decorative printing layer 23 is used to draw characters or slit patterns 26. When drawing some characters or patterns, they can be seen from an opposite side of the base plate 22 since the base plate 22 is transparent. The electrode layer 24 is a membrane of an electrically conductive ink (e.g., an electrically conductive ink containing carbon graphite, polyester resin and solvent by a weight ratio of 30%, 25 and 45% respectively) printed on the decorative printing layer 23 at a thickness of about 0.021 mm. The electrode layer 24 has plural rectangular electrodes 21 printed with electrically conductive ink along a longitudinal direction of the main scale 20. The shape of the electrode 21 can be defined by a long side of which dimension is almost the same as the width of the base plate 22 and a short side having a predetermined length. The bonding layer 25 is of a membrane of a bonding agent entirely applying over the electrode layer 24 by a thickness of about 0.16 mm. The applicable bonding agent is for example a double-adhesive tape. The bonding agent is much supplied into spaces disposed between nearby electrodes 24 or portions other than the electrodes 21 of the electrode layer 24 until reaching to the surface of the decorative printing layer 23. Now referring to FIG. 4, the electrical capacitance encoder 40 is depicted in detail. The electrical capacitance encoder 40 is defined by the above explained lattice electrodes 21 which are printed successively on the base scale 1 every certain intervals, earth ground electrode (not shown) electrically arranged between these electrodes 21, plural sets of sending electrodes 41 provided in the slider 10 each of which is composed with eight electrodes arranged at certain intervals along the base scale 1, a receiving electrode 42 arranged along a series of the sending electrodes 41 in a web state, a sending circuit 43 to impress into the sending electrodes 41 signals dividing 360 degrees into eight different phases, a receiving circuit 44 to receive signals from the receiving electrode 42 presenting different phase based on an instant displacement value between the sending electrodes 41 and the electrodes 21. An output from the receiving circuit 44 or originally issued from the electrical capacitance encoder 40 is counted in a counter 45 and then diplayed in digital by means of the digital display 14 after a predetermined processing in a CPU 46. Incidentally, the counted value in the counter 45 can be reset at "0" value when the zero-setting switch 16 is pressed. If necessary, the measured value may be printed out or further processed through a transmitter 47 in an external printer by sending data processed in the CPU 46 through a cable or by radio. In measurement, the slider 10 is moved to measure an object to be measured from a situation that the inboard measuring jaws 2 and 12 of the base scale 1 and the slider 10 are contacting to each other. The displacement value of the slider 10 can be detected by the electrical capacitance encoder 40 and is then displayed in digital by means of the digital display 14. Accordingly, the measurement in the above mentioned first embodiment is performed simply by abutting the inboard measuring jaws 2 and 12 12 or the outboard measuring jaws 3 and 13 to both ends of the object to be measured respectively to obtain measured value on the digital display 14. When measuring inner diameter of a hole provided in the object to be measured, the measuring jaws 2 and 12 are respectively abutted an inner surface of the hole to display the measured value in digital by means of the digital display 14, which should be an easy measuring work for an operator. When measuring a thickness of the object to be measured, the outboard measuring jaws 3 and 13 are utilized such that they are abutted to a portion to be measured to thereby obtain the objective thickness in digital by the digital display 14. In manufacturing, plenty of main scales 20 are printed on a large polyester sheet all together and then covered entirely with the bonding agent to form the bonding layer 25, so that the respective main scales 20 can be obtained as required by cutting the large polyester sheet. In this process, the surface of the bonding layer 25 of the main scale 20 is preferably shielded with a temporary sheet cover until being attached on to the base scale 1 to secure bonding layer 25 to ensure a firm attachment of the main scale 20. As has been mentioned, the base scale 1 and the slider 10 are manufactured by means of the injection molding machine wherein the engineering plastics are consumed as materials. On the completed base scale 1, the main scale 20 is attached securely and then the base scale 1 is combined with the slider 10. The slider calipers according to the present invention will be completed after fine adjustments for several combined parts. The example according to the present invention is merited in developing a high-volume productivity of the main scales 20, as plural main scales 20 can be manufactured by printing electrodes 21 all at once on the large film of an insulator film. Since the base scale 1 and the slider 10 are made of engineering plastics which can be injected from a mold, a desirable high-volume productivity of the base scale 1 and the slider 10 can be achieved. As the bonding layer 25 is formed by applying the bonding agents on the surface where the electrodes 21 of the base plate 22 are made in advance before attaching the main scale 20 to the base scale 1, the attachment work of the main scale 20 toward the main body 1 can be proceeded smoothly by means of the bonding layer 25 and there is no necessity to provide an insulation layer additionally because of a mutual insulation state of the respective electrodes 21. The fine attachment work of the main scale 20 to the base scale 1 may be performed as the base plate 22 is of a film having plasticity. According to the combination of the transparent base plate 22 and the electrode layer 24 in a state that the decorative printing layer 23 are intervened between them, any character or pattern 26 printed on the decorative printing layer 23 can be seen clearly through the base plate 22. The base scale 1 and the slider 10 are both made of engineering plastics so that the base scale 1 can be kept in an insulated state. It will not be required to have the insulation layer between the base scale 1 and the main scale 20 to obtain insulation properties between nearby electrodes 21 of the main scale 20 attached on the base scale 1. Furthermore, the insulation properties between the electrodes 21 can be improved due to the mentioned bonding layer 25 for the main scale 20. Since the electrodes 21 are formed by painting and the base scale 1 and the slider 10 are made of engineering plastics, the high-volume productivity of respective components is improved. As the bonding layer 25 brings a result on an attachment work to the main scale 20, the productivity of the slide calipers can be extremely improved to thereby achieve reduced cost. As the base scale 1 is made of engineering plastics, a further lightweighting can be achieved to thereby improve a comfortable operation and handling. In the above mentioned example, there is explained the slide calipers to measure a straight distance such as inner diameter, length or the like by moving the slider 10 of the measuring element in a sliding contact state with the base scale 1, but the present invention can be applied also to a rotation angle measuring instrument such as a rotary encoder to measure an angle of rotation degrees when a measuring element and a main scale are moved relatively to each other. When applying to the rotation angle measuring instrument, the main scale should be formed, as shown in FIG. 5, into a disc-shaped base plate 32 made of synthetic resin film with plural electrodes 31 printed thereon by predetermined intervals angularly. The material for the base scale 1 and the slider 10 are not only to be engineering plastics composed with PBT resin and additional mineral and CFRP but carbon fiber reinforced plastic using the base material such as nylon resin having stiffness properties to which a fiber such as glass are supplied. Incidentally, the base material and the supplied material may be naturally other resins and fibers. Taking for an instance for the base material, there are applicable to be used ABS resin and jyuracon (polyacetal resin) having high wear-resistant properties. The supplied material may be a single fiber such as glass fiber or KEVLAR (trademark) otherwise a composite fiber thereof. Referring to the electrically conductive ink used for printing the electrodes 21, it contains carbon graphite and polyester resins in the above mentioned embodiments but may be made of carbon paste and medium or made of silver paste. The base plate 22 for the main scale is not only limited to be one made of polyester but other synthetic resin such as triacetate, polystyrene or the like. It should be naturally understood that the present invention can be applied to other apparatus than the slide calipers, for example, a measuring apparatus such as height gauge and a feeding value displaying apparatus for a machine tool such as drilling machine. If the main scale is formed into a web state and then applied on a similar web shaped plastic member made of metal or synthetic resin, a tape measure displaying the measured value in digital can be provided. As has been mentioned above, in accordance with the present invention, desirable high-volume productivity and reduced cost can be achieved.

Claims: An electrical capacitance instrument having a main scale on a main body (1) and a measuring element (10) which is arranged to move in relation to the main body to provide a measurement of a displacement value of the measuring element relative to the main body; wherein the main scale is provided by an insulating base plate (22), and a plurality of electrodes (21) printed with an electrically conductive ink on the base plate at intervals along the direction of movement of the measuring element; and characterised by an insulating bonding layer (25) formed over the electrodes and between the base plate and the main body and bonding the main scale to the main body. An instrument according to claim 1, wherein the base plate (22) is made of a transparent material. An instrument according to claim 1 or claim 2, wherein the base plate (22) is made of a film having plasticity. An instrument according to any one of the preceding claims, wherein the main scale includes a decorative layer (23) printed with characters or patterns, and positioned between the base plate (22) and an electrode layer (24) of the printed electrodes. An instrument according to any one of the preceding claims, wherein the main body (1) is made of a material having insulation properties. An instrument according to any one of the preceding claims, wherein the main body (1) is made of engineering plastics. An instrument according to any one of the preceding claims, wherein the main body (1) has at least one measuring jaw (2, 3), and the measuring element (10) is arranged to move along the main body in a sliding contact state with the main scale and has at least one measuring jaw (12,13) corresponding to that (those) of the main scale. A method of manufacturing an electrical capacitance instrument having a main body (1) with a main scale, and a measuring element (10) which is arranged to move in relation to the main body to provide a measurement of a displacement value of the measuring element relative to the main body; the method comprising printing a plurality of electrodes (21) on a base plate (22) made of an insulating film at intervals with an electrically conductive ink to form the main scale; characterised by forming an insulating bonding layer (25) on the base plate and covering the electrodes; and bonding the base plate to the main body by means of the bonding layer so that the bonding layer is between the base plate and the main body. A method according to claim 8, wherein the base plate (12) is made of a transparent material, and a decorative printed layer (23) is formed on the base plate, and the electrodes being printed on the decorative layer. A method according to claim 8 or claim 9, wherein the main body (22) is formed by injection moulding.

Inventors: KOIZUMI HIROSHI, SHIRAI TOSHIYUKI, SUZUKI TAMIO, KOIZUMI, HIROSHI, SHIRAI, TOSHIYUKI, SUZUKI, TAMIO
Applicants: MITUTOYO CORP, MITUTOYO CORPORATION
Application year: 1994
Main classification: G01D5
IPC codes: G01B3, G01D5